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- Genre: Academic theses
- Creators: Tasooji, Amaneh
- Creators: Chamberlin, Ralph
- Creators: Holcomb, Don
Description
The heat transfer enhancements available from expanding the cross-section of a boiling microchannel are explored analytically and experimentally. Evaluation of the literature on critical heat flux in flow boiling and associated pressure drop behavior is presented with predictive critical heat flux (CHF) and pressure drop correlations. An optimum channel configuration allowing maximum CHF while reducing pressure drop is sought. A perturbation of the channel diameter is employed to examine CHF and pressure drop relationships from the literature with the aim of identifying those adequately general and suitable for use in a scenario with an expanding channel. Several CHF criteria are identified which predict an optimizable channel expansion, though many do not. Pressure drop relationships admit improvement with expansion, and no optimum presents itself. The relevant physical phenomena surrounding flow boiling pressure drop are considered, and a balance of dimensionless numbers is presented that may be of qualitative use. The design, fabrication, inspection, and experimental evaluation of four copper microchannel arrays of different channel expansion rates with R-134a refrigerant is presented. Optimum rates of expansion which maximize the critical heat flux are considered at multiple flow rates, and experimental results are presented demonstrating optima. The effect of expansion on the boiling number is considered, and experiments demonstrate that expansion produces a notable increase in the boiling number in the region explored, though no optima are observed. Significant decrease in the pressure drop across the evaporator is observed with the expanding channels, and no optima appear. Discussion of the significance of this finding is presented, along with possible avenues for future work.
ContributorsMiner, Mark (Author) / Phelan, Patrick E (Thesis advisor) / Baer, Steven (Committee member) / Chamberlin, Ralph (Committee member) / Chen, Kangping (Committee member) / Herrmann, Marcus (Committee member) / Arizona State University (Publisher)
Created2013
Description
Recent literature indicates potential benefits in microchannel cooling if an inlet orifice is used to suppress pressure oscillations that develop under two-phase conditions. This study investigates the costs and benefits of using an adjustable microchannel inlet orifice. The focus is on orifice effect during steady-state boiling and critical heat flux (CHF) in the channels using R134a in a pumped refrigerant loop (PRL). To change orifice size, a dam controlled with a micrometer was placed in front of 31 parallel microchannels. Each channel had a hydraulic diameter of 0.235 mm and a length of 1.33 cm. For steady state two-phase conditions, mass fluxes of 300 kg m-2 s-1 and 600 kg m-2 s-1were investigated. For orifice sizes with a hydraulic diameter to unrestricted hydraulic diameter (Dh:Dh,ur) ratio less than 35 percent, oscillations were reduced and wall temperatures fell up to 1.5 °C. Critical heat flux data were obtained for 7 orifice sizes with mass fluxes from 186 kg m-2 s-1 to 847 kg m-2 s-1. For all mass fluxes and inlet conditions tested, CHF values for a Dh:Dh,ur ratio of 1.8 percent became increasingly lower (up to 37 W cm-2 less) than those obtained with larger orifices. An optimum orifice size with Dh:Dh,ur of 35 percent emerged, offering up to 5 W cm-2 increase in CHF over unrestricted conditions at the highest mass flux tested, 847 kg m-2 s-1. These improvements in cooling ability with inlet orifices in place under both steady-state and impending CHF conditions are modest, leading to the conclusion that inlet orifices are only mildly effective at improving heat transfer coefficients. Stability of the PRL used for experimentation was also studied and improved. A vapor compression cycle's (VCC) proportional, integral, and derivative controller was found to adversely affect stability within the PRL and cause premature CHF. Replacing the VCC with an ice water heat sink maintained steady pumped loop system pressures and mass flow rates. The ice water heat sink was shown to have energy cost savings over the use of a directly coupled VCC for removing heat from the PRL.
ContributorsOdom, Brent A (Author) / Phelan, Patrick E (Thesis advisor) / Herrmann, Marcus (Committee member) / Trimble, Steve (Committee member) / Tasooji, Amaneh (Committee member) / Holcomb, Don (Committee member) / Arizona State University (Publisher)
Created2012
Description
The microstructure development of Inconel alloy 718 (IN718) during conventional processing has been extensively studied and much has been discovered as to the mechanisms behind the exceptional creep resistance that the alloy exhibits. More recently with the development of large scale 3D printing of alloys such as IN718 a new dimension of complexity has emerged in the understanding of alloy microstructure development, hence, potential alloy development opportunity for IN718.
This study is a broad stroke at discovering possible alternate microstructures developing in Direct-Metal-Laser-Sintering (DMLS) processed IN718 compared to those in conventional wrought IN718. The main inspiration for this study came from creep test results from several DMLS IN718 samples at Honeywell that showed a significant
improvement in creep capabilities for DMLS718 compared to cast and wrought IN718 (Honeywell).
From this data the steady-state creep rates were evaluated and fitted to current creep models in order to identify active creep mechanisms in conventional and DMLS IN718 and illuminate the potential factors responsible for the improved creep behavior in DMSL processed IN718.
Because rapid heating and cooling can introduce high internal stress and impact microstructural development, such as gamma double prime formations (Oblak et al.), leading to differences in material behavior, DMLS and conventional IN718 materials are studied using SEM and TEM characterization to investigate sub-micron and/or nano-scale
microstructural differences developed in the DMLS samples as a result of their complex thermal history and internal stress.
The preliminary analysis presented in this body of work is an attempt to better understand the effect of DMLS processing in quest for development of optimization techniques for DMLS as a whole. A historical sketch of nickel alloys and the development of IN718 is given. A literature review detailing the microstructure of IN718 is presented. Creep data analysis and identification of active creep mechanisms are evaluated. High-resolution microstructural characterization of DMLS and wrought IN718 are discussed in detail throughout various chapters of this thesis. Finally, an initial effort in developing a processing model that would allow for parameter optimization is presented.
This study is a broad stroke at discovering possible alternate microstructures developing in Direct-Metal-Laser-Sintering (DMLS) processed IN718 compared to those in conventional wrought IN718. The main inspiration for this study came from creep test results from several DMLS IN718 samples at Honeywell that showed a significant
improvement in creep capabilities for DMLS718 compared to cast and wrought IN718 (Honeywell).
From this data the steady-state creep rates were evaluated and fitted to current creep models in order to identify active creep mechanisms in conventional and DMLS IN718 and illuminate the potential factors responsible for the improved creep behavior in DMSL processed IN718.
Because rapid heating and cooling can introduce high internal stress and impact microstructural development, such as gamma double prime formations (Oblak et al.), leading to differences in material behavior, DMLS and conventional IN718 materials are studied using SEM and TEM characterization to investigate sub-micron and/or nano-scale
microstructural differences developed in the DMLS samples as a result of their complex thermal history and internal stress.
The preliminary analysis presented in this body of work is an attempt to better understand the effect of DMLS processing in quest for development of optimization techniques for DMLS as a whole. A historical sketch of nickel alloys and the development of IN718 is given. A literature review detailing the microstructure of IN718 is presented. Creep data analysis and identification of active creep mechanisms are evaluated. High-resolution microstructural characterization of DMLS and wrought IN718 are discussed in detail throughout various chapters of this thesis. Finally, an initial effort in developing a processing model that would allow for parameter optimization is presented.
ContributorsRogers, Blake Kenton (Author) / Tasooji, Amaneh (Thesis advisor) / Petuskey, William (Committee member) / Rogers, Bradley (Committee member) / Arizona State University (Publisher)
Created2017